1. Field
The present invention relates generally to laser systems. More specifically, the invention relates to estimating a concentration of reactive gas into the chambers of a gas discharge laser, such as a two chamber Master Oscillator-Power Amplifier excimer laser.
2. Description of Related Art
One type of gas discharge laser used in photolithography is known as an excimer laser. An excimer laser typically uses a combination of a noble gas, such as argon, krypton, or xenon, and a reactive halogen gas such as fluorine or chlorine. The excimer laser derives its name from the fact that under the appropriate conditions of electrical stimulation and high pressure, a pseudo-molecule called an excimer (or in the case of noble gas halides, an exciplex) is created, which can only exist in an energized state and can give rise to laser light in the ultraviolet range.
Excimer lasers are widely used in high-resolution photolithography machines, and are thus one of the critical technologies required for microelectronic chip manufacturing. Current state-of-the-art lithography tools use deep ultraviolet (DUV) light from the KrF and ArF excimer lasers with nominal wavelengths of 248 and 193 nanometers respectively.
While excimer lasers may be built with a single chamber light source, the conflicting design demands for more power and reduced spectral bandwidth have meant a compromise in performance in such single chamber designs. One way of avoiding this design compromise and improving performance is by utilizing two chambers. This allows for separation of the functions of spectral bandwidth and pulse energy generation; each chamber is optimized for one of the two performance parameters.
Such dual-gas-discharge-chamber excimer lasers are often called Master Oscillator-Power Amplifier, or “MOPA,” lasers. In addition to improving the spectral bandwidth and pulse energy, the efficiency of the dual chamber architecture can enable the consumable modules in MOPA lasers to reach longer operational lifetimes than their counterpart modules in single chamber light sources.
In each chamber, as the light source discharges energy across its electrodes to produce light, the halogen gas, fluorine in the case of ArF or KrF lasers, is depleted. This causes a decrease in the laser efficiency which is seen, for example, as an increase in discharge voltage required to create a given desired pulse energy. Since the discharge voltage has an upper limit determined by physical constraints of the hardware, steps must be taken to replenish the lost fluorine so that the voltage remains below this limit and the laser continues to function properly.
In order to replenish the lost fluorine, the amount of fluorine remaining in the chambers is estimated. Sensors in the MOPA system may measure a number of variables in the system but these measurements, without further refinement, are known to be inaccurate for calculating the current fluorine concentration. Various algorithms have been proposed to correct for these inaccuracies including adding a forgetting factor to the calculations to weigh more recent measurements greater than older measurements. However, in the event of a mode change, the use of the forgetting factor may delay the response of the MOPA system to the mode change.